Colloidal semiconductor nanocrystals is an important field in modern nanoscale science and technology, see Bawendi, M. G.; Steigerwald, M. L.; Brus, L. E. Annu. Rev. Phys. Chem. 1990, 41, 477-496 and Alivisatos, A. P. Science 1996, 271, 933-937, the contents of both are hereby incorporated by reference in their entirety for all purposes. Among the various materials, colloidal CdSe quantum dots are undoubtedly the most studied, due to their tunable emission in the visible range, the advances in their preparation and their potential use in industrial and biomedical applications.
Recently, several advances in the synthesis of colloidal semiconductor nanocrystals have been made, allowing for size and shape control, see Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59-61 and Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700-12706, the contents of both are hereby incorporated by reference in their entirety for all purposes. Of particular interest in this respect is the ability to obtain quantum confined wurtzite CdSe nanorods with a narrow distribution of lengths and diameters. Well-characterized samples of CdSe nanorods have become a model system to study theories of quantum confinement: for instance, it has been demonstrated, both theoretically and experimentally, that they emit linearly polarized light along the c-axis and that the degree of polarization is dependent on the aspect ratio of the particles. Semiconductor nanorods are of particular interest because of their possible applications in light emitting diodes, in low-cost photovoltaic devices, their propensity to form liquid crystalline phases and their use as barcodes for analytical purposes.
U.S. Pat. No. 6,225,198, the contents of which are hereby incorporated by reference in its entirety for all purposes discloses processes for forming Group II-VI semiconductor nanocrystals and rod-like structures by contacting the semiconductor nanocrystal precursors with a liquid media comprising a binary mixture of phosphorous-containing organic surfactants. In semiconductor quantum dots, which are nanocrystals and not the nanorods of the present invention, high emission efficiency from band-edge states is required to study in detail their electronic structure or more practically, if they are to be used as emitters in any application. Unfortunately, the band-edge emission from nanocrystals has to compete with both radiative and non-radiative decay channels, originating from surface electronic states. In colloidal nanocrystals, coating the surface of the nanocrystals with suitable organic molecules can minimize this problem. The judicious choice of a passivating agent can in fact improve the size-dependent band-edge luminescence efficiency, while preserving the solubility and processability of the particles. Unfortunately, passivation by means of organic molecules is often incomplete or reversible, exposing some regions of the surface to degradation effects such as photooxidation. In some cases, chemical degradation of the ligand molecule itself or its exchange with other ligands might lead to unstable and therefore unusable nanocrystals.
In the case of colloidal CdSe nanorods, there are two additional factors that might further reduce the luminescence from band-edge states, when compared to spherical CdSe nanocrystals. In nanorods, the surface-to-volume ratio is higher than in spheres, and this increases the occurrence of surface trap-states. In larger dots, the increased delocalization of carriers reduces the overlap of the electron and hole wavefunctions, lowering the probability of radiative recombination. The delocalization of carriers should be particularly high in a nanorod, where they are free to move throughout the length of the rod, thereby leading to reduced luminescence in nanorods. In order to efficiently and permanently remove most of the surface states of the nanocrystal, an inorganic material can be epitaxially grown on its surface, see Peng, X. G.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019-7029, the contents of which is hereby incorporated by reference in their entirety for all purposes.
A stringent requirement for the epitaxial growth of several monolayers of one material on the top of another is a low lattice mismatch between the two materials. If this requirement is not met, a strain accumulates in the growing layer and eventually it may be released through the formation of misfit dislocations, degrading the optical properties of the system, see for example Dabbousi, B. O.; Rodriguez Viejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101, 9463-9475, the contents of which is hereby incorporated by reference in their entirety for all purposes. The preparation of a coated semiconductor nanocrystal may be found in U.S. Pat. Nos. 6,607,829 and 6,322,901 the contents of which are all incorporated by reference in their entirety for all purposes.
In the case of “spherical” colloidal CdSe nanocrystals, there are two methods of efficient inorganic passivation, one by means of a spherical layer (or shell) of ZnS, and the other by means of a shell of CdS. The choice of these materials is based on the fact that both ZnS and CdS provide a potential step for electrons and holes originating in the nanocrystals, reducing the probability for the carriers to sample the surface. Surprisingly, the requirement for a low lattice mismatch is not as stringent as for 2D systems, because the total area over which the strain accumulates is small, and the total strain energy at the interface can remain below the threshold for inducing dislocations. The extended surface of the CdSe rods has an average curvature that is intermediate between the surface of a spherical dot and that of a flat film. In addition, since CdSe nanorods can be produced with lengths ranging from a few nanometers to a hundred nanometers, the coherent growth of an epitaxial shell over a region that is much more extended than the surface of a spherical dot is more challenging. Both conditions imply that interfacial strain will play a much more important role in rods than in dots. An additional issue that must be taken into account is the solubility of the resulting particles. The shell growth must be carried out in a surfactant that provides surface accessibility for the shell material to grow, while preventing aggregation of the particles. The temperature must also be kept low enough to prevent nucleation of the shell material, while high enough that the surfactant is dynamically going on and off the nanocrystal surface allowing access to the monomers.
The art is replete with chemical and biological assays to identify a particular analyte of interest. Examples included immunoassays, fluorescence, signal amplification, nucleic acid hybridization and high throughput screening. Each of the above-described assay formats utilizes detectable labels to identify the analyte of interest. Radio-labeled molecules and compounds are frequently used to detect biological compounds both in vivo and in vitro. However, due to the inherent problems associated with the use of radioactive isotopes, non-radioactive methods of detecting biological and chemical compounds are often preferable. U.S. Pat. No. 6,274,323 (the contents of which are hereby incorporated by reference in its entirety for all purposes) discloses the use of semiconductor nanocrystals, or quantum dots in a barcode system for identification. The disadvantage of this technology is that multiple quantum dots are required to perform an assay.